The first is two volumes’ worth of compiled Math Olympiad problems. I’d forgotten about these, but I like them because there’s usually more than one way to solve each problem and the solver doesn’t know what kind of problem to expect at the outset. You can see sample problems here; they come in “elementary” and “middle school” levels. (It may look as though I’ve botched the links, but I haven’t. If you order any, be careful to examine the covers before choosing. Volume 1 is called “Contest Problems for Elementary and Middle School” but it only contains elementary level problems. Volume 2 is called “Contest Problems” but has some of each level. I think it’s just because when the first volume came out there was no second volume, and the title was describing the program, not the contents of the book.)

The other is a series of beautifully designed books from Thunder Bay Press called Doodle Yourself Smart. There’s a math volume, a geometry volume, and a physics volume. Each book has 100 or so puzzles, one per page with lots of room for figuring (and doodling, ostensibly). They’re refreshingly well-designed, for math books. The pages are actually pleasant to look at. I’ve only interacted directly with the math volume (the puzzle I did was about finding pairs of primes that added up to various target numbers), but I’m looking forward to the other two. I’d recommend these especially for math-likers with aesthetic sensibilities.